The subject of the present invention is a brake booster, more particularly of the type comprising a housing in which is displaced a driving piston having a hub and separating a front chamber from a rear chamber, the front chamber being connected to a source of fluid under a first pressure, the rear chamber being capable of being connected to a source of fluid under a second pressure, and the driving piston being capable of being displaced from a rest position in response to the establishment of a difference between the first and second pressures between the two chambers, the pressure difference being controlled by valve means associated with input control members.
Such boosters are well known in the art of braking and function perfectly. The most frequent instance of use is encountered when the front chamber is connected to a vacuum source and the rear chamber is connected to a vacuum source and the rear chamber is capable of being connected to a source of air at atmospheric pressure during the actuation of the valve means.
However, there are instances where it is desirable, rather than using air at atmospheric pressure, to use air under a higher pressure, for example compressed air, in order to increase the boosting effect, reduce the overall size of the booster, etc.
Various solutions have been provided in this respect. Thus, U.S. Pat. No. 3,780,620 describes a booster in which the valve means comprise a plunger which is located in the hub of the piston and which can occupy three positions. In the first, it allows communication between the front chamber and the rear chamber by opening a passage in the valve means. In the second, the plunger shuts off this passage and allows air at atmospheric pressure to penetrate into the rear chamber via the valve means and thus generate a pressure difference on the driving piston, thereby providing the desired boost. In the third, the plunger engages second valve means in order to admit air under a pressure higher than atmospheric pressure into the rear chamber. Such an arrangement is very complicated and therefore very expensive and unreliable.
Again, U.S. Pat. No. 4,ll7,769 makes known a booster which possesses, inside the rear chamber, an auxiliary driving member comprising a pressure chamber capable of being connected to an external pressure source by means of a control device. When the control device is actuated, the auxiliary driving member thus exerts on the driving piston of the booster an auxiliary boosting force completely independent of the boosting force brought about by the actuation of the brake pedal. In the document U.S. Pat. No. 4,117,769, this auxiliary boosting force is utilized completely independently of the normal braking system of the vehicle in order to make it possible to add an assisted automatic braking system to this, without the need to resort to an additional booster. Such a system therefore requires an additional control device and a considerable modification of the booster in order to include the auxiliary driving member in it.